Recently, organic photoconductors (OPC) have been replacing inorganic photoconductor for their excellent performance and various advantages, and are often applied to copiers, facsimile machines, laser printers and complex machines thereof. Examples of the reasons for this include (1) optical property such as a wide range of the wavelength of light absorption and a large amount of light absorption, (2) electric property of high sensitive and stable charging property, (3) a wide range of material selection, (4) easiness to produce, (5) low cost, and (6) non-toxicity.
As reducing the diameter of a photoconductor is progressed by downsizing of image forming apparatuses recently and high-speed movements and maintenance-free of apparatuses are followed, highly durable photoconductors are being desired. Viewed from this point, as a surface layer of the organic photoconductor contains mainly low molecular charge transport materials and inactive polymers, the organic photoconductor is generally soft. Because of this chemical property, the organic photoconductor has a disadvantage of frequent wearing caused by mechanical overload through developing systems or cleaning systems, when the organic photoconductor is repeatedly used in the electrophotography process. Furthermore, because of increasing demand of high image quality, rubber hardness and contact pressure of cleaning blades are increased for the purpose of improving cleaning with the trend of reducing the diameter of toner particles, and such a requirement is a cause for accelerating the wear of the photoconductor. Thus wear of the photoconductor impairs sensitivity and electric property such as lowering of charging, and causes lowering of image densities and abnormal images of dirty backgrounds. Scratches due to localized wears cause striped-dirt images due to defective cleaning. The exhaustion of the life of the photoconductor is ratio-determined by wears and scratches and thereby the photoconductor are led to the replacement in the present condition.
Thus, for enhancing the durability of the organic photoconductor (OPC), it is indispensable to lower wear degree and it is in need of organic photoconductors that not only have a fine surface for superior cleaning and adding transferring but also have no long-term dependencies of places over electrophotographic property and maintain stable high performance. For this reason, this is the most urgent problem to be solved in the art.
Examples of the technology for improving wear resistance property of the photosensitive layer include (1) a method for using curable binder in a surface layer (see Patent Literature 1), (2) a method for using a high-molecular weight charge transport material in a surface layer (see Patent Literature 2) and (3) a method for using inorganic fillers dispersed in a surface layer (see Patent Literature 0.3). Among these methods, the surface layer described in the method (1) has a tendency of lowering the image density as residual potential is elevated by poor compatibility of the curable binder with charge transport materials and the presence of impurities such as a polymerization initiator and unreacted residues. Although both the surface layer described in the method (2) that contains a charge transportable polymer material and the surface layer described in the method (3) that contains dispersed inorganic fillers can improve wear resistance property to some extents, the current situation is that fully satisfactory durability required for organic photoconductors has not yet been obtained. Additionally, the surface layer described in the method (3) has a tendency of flowering image densities as residual potential is elevated by charge traps that exist on the inorganic filler surface. For this reason, any of these methods (1), (2), and (3) has not yet succeeded in fully achieving overall durability, including electric durability and mechanical durability that are required for organic photoconductors.
For improving wear resistance property and scratch resistant property of the surface layer described in the method (1), a photoconductor containing multi-functional curable acrylate monomers is proposed (see Patent Literature 4). Although this Patent Literature discloses a photoconductor in which its protective layer (or surface layer) disposed on the photosensitive layer contains the multi-functional curable acrylate monomer, it merely describes the fact that the protective layer may contain a charge transport material and fails to provide a specific description. Furthermore, when a low molecular weight charge transport material is simply contained in the protective layer, its compatibility with the cured material of the foregoing monomer becomes a problem. As a result, this may cause deposition of the low-molecular weight charge transport material and cracking in the surface layer, and finally lowering its mechanical strength. This Patent Literature also discloses that a polycarbonate resin is contained in the surface layer for increased compatibility; however, this causes a reduction in the content of the curable acrylic monomer and thus a sufficient wear resistance has not yet been obtained with this method. With regards to a photoconductor with no charge transport materials in the surface layer, the Patent Literature discloses that the surface layer is made thin for decreased exposed area potential, this photoconductor, however, has a short life because of the thin surface layer. Besides, the environmental stability of the charging potential and the exposed area potential is poor, and the values of the charging potential and the exposed area potential significantly fluctuate substantially depending on the environmental temperature and humidity, thereby failing to maintain sufficient values.
As an alternative wear resistance technology for the photosensitive layer, a method for using coating solution containing monomers having a carbon-carbon double bond, charge transport materials having a carbon-carbon double bond, and binder resins to form a charge transport layer is proposed (see Patent Literature 5). The proposed binder resin is classified into two types: one reactive to the charge transport materials having a carbon-carbon double bond and one not reactive to the charge transport materials having no carbon-carbon double bond. The photoconductor draws attention because of the simultaneous achievement of wear resistance property and superior electric property; however, when a non-reactive binder resin is used, the compatibility of the binder resin with the cured material produced by reaction of the monomer with the charge transport material becomes poor, surface unevenness occurs due to layer separation at the time of cross-linking, thereby causing the tendency of defective cleaning. In this case, specifically described one that not only prevents the binder resin from monomer curing and but also is used for producing a photoconductor is a bifunctional monomer; however, this bifunctional monomer has a small number of functional groups, thus resulting in failure to obtain a sufficient cross-linkage density and thereby wear resistance property is not yet satisfactory. Moreover, even in the case where a reactive binder is used, due to a small number of functional groups contained in the monomer and the binder resin, the simultaneous achievement of the bond amount of the charge transport materials and cross-linkage density becomes difficult, and thereby electric property and wear resistance property of the photoconductor are not satisfactory.
Besides, the photosensitive layer containing a compound of a cured hole transportable compound having two or more chain polymerizable functional groups in the same molecule is proposed (see Patent Literature 6). However, the photosensitive layer of the proposition generates strain within a curable because a bulky hole transportable compound has two or more chain polymerizable functional groups, enhances an internal stress, tends to generate surface layer roughness, and cracking over time, thereby failing to achieve sufficient durability.
Besides, the electrophotographic photoconductor having cured cross-linked layer of a radically polymerizable compound having three or more functionalities with no charge transport structure and a radically polymerizable compound having single functionality with charge transport structure is proposed (see Patent Literatures 7 to 20 for example). In these propositions, using a monofunctional radically polymerizable compound with charge transport structure controls mechanical and electrical durability and generation of cracking in the photosensitive layer. However, in case of forming this cross-linked layer, an acrylic monomer having a multiple number of acrylic functional groups is cured to achieve high wear resistance. In this case, the acrylic cured material significantly shrinks in volume; thereby adhesiveness with photosensitive layer, that is, a lower layer may become insufficient. Besides, when an image forming apparatus that poses a high mechanical hazard to the electrophotographic photoconductor is used, there is an issue of yielding peeling of the cross-linked layer and the electrophotographic photoconductor cannot maintain sufficient wear resistance for prolonged periods. There is no sufficient description about the photoconductor temperature during curing for the formation of the cross-linked layer, but there is only disclosed information of controlling the photoconductor temperature at the time of exposure so as not to exceed 50° C.; however, sufficient curing at around 50° C. of the photoconductor temperature may not be expected and there is no description of controlling photoconductor temperature controlling method, thus there is no way but to shorten the exposure for preventing the photoconductor temperature from exceeding 50° C. However, if the exposure time is shortened, promotion of sufficient polymerization reaction may not be expected, thereby high wear resistance for prolonged periods cannot be maintained. Furthermore, in case of sufficient polymerization reaction, there is no discussion about evenness of the photoconductor temperature. Homogeneous polymerization of the cross-linked layer is undone with subdued difference between maximum value and minimum value of the post-exposure electrical potential, and thereby stable photoconductor property for prolonged periods cannot be achieved.
Besides, there are proposals in which a prescribed photoconductor temperature at the time of exposure is set by forming a cross-linked surface layer by curing of a photopolymerizable monomer (see Patent Literatures 21 and 22). These propositions have no detailed explanation about the method for controlling temperature, but only description of temperature being controlled by air cooling in Examples; however, if air is used as coolant media, cooling efficiency becomes very low because of its low thermal conductivity, amount of heat which is generated by curing with powerful irradiation light cannot be reduced, longtime exposure becomes impossible, and thereby sufficient polymerization reaction is not completed. Besides, in case of method for controlling temperature, fluctuation of flow rate and cooling efficiency by method becomes bigger and thereby cured level of a cross-linked surface layer fluctuates. That is, the dependency of places of wear resistance and electric property is large, the difference between maximum value and minimum value of the post-exposure electrical potential with respect to electric property cannot be stemmed, and thereby stable property for prolonged periods cannot be maintained.
Consequently, any of electrophotographic photoconductors having a cross-linked layer which is chemically bonded with charge transport structure in these conventional technologies has not yet provided sufficient total property in the present state of affairs.    [Patent Literature 1] Japanese Patent Application Laid-Open (JP-A) No. 56-48637    [Patent Literature 2] JP-A No. 64-1728    [Patent Literature 3] JP-A No. 04-281461    [Patent Literature 4] Japanese Patent (JP-B) No. 3262488    [Patent Literature 5] JP-B No. 3194392    [Patent Literature 6] JP-A No. 2000-66425    [Patent Literature 7] JP-A No. 2004-302450    [Patent Literature 8] JP-A No. 2004-302451    [Patent Literature 9] JP-A No. 2004-302452    [Patent Literature 10] JP-A No. 2005-099688    [Patent Literature 11] JP-A No. 2005-107401    [Patent Literature 12] JP-A No. 2005-107490    [Patent Literature 13] JP-A No. 2005-115322    [Patent Literature 14] JP-A No. 2005-140825    [Patent Literature 15] JP-A No. 2005-156784    [Patent Literature 16] JP-A No. 2005-157026    [Patent Literature 17] JP-A No. 2005-157297    [Patent Literature 18] JP-A No. 2005-189821    [Patent Literature 19] JP-A No. 2005-189828    [Patent Literature 20] JP-A No. 2005-189835    [Patent Literature 21] JP-A No. 2001-125297    [Patent Literature 22] JP-A No. 2004-240305